Enhanced pdbtrack to provide for source code that's not findable by

the reported path.  (Eg, precompiled scripts with a file path suitable
for a different host, scripts actually running on a remote system or
with no valid path, like Zope through-the-web python scripts.)

On failing to find the code on the reported path, pdbtrack takes the
function name and looks through the buffers, from most to least
recent, seeking the first python-mode buffer that either is named for
the function or has a definition (def or class) for that function.  So
to get source tracking for code that's not located where the path
indicates, you put a copy of the script in a buffer, and pdbtrack will
find it.

Also, fixed a small bug so pdbtrack now properly presents the overlay
arrow when you run the pdb 'w'here command.